Although useful in many commercial applications, polyolefin homopolymers, such as high density polyethylene (HDPE) and isotactic polypropylene (i-PP), suffer poor interaction with other materials. The inert nature of polyolefins significantly limits their end uses, particularly those in which adhesion, dyeability, paintability, printability or compatibility with other functional polymers is paramount.
Unfortunately, because of their inert nature and crystallinity, polyolefins have been among the more difficult materials to chemically modify by means of post-polymerization processes. In many cases, the post-polymerization reactions result in serious side reactions, such as degradation and crosslinking reactions. Although the direct copolymerization is the most effective route to functionalize polyolefins, such direct processes usually are laden with difficulties and limitations.
Only the transition metal coordination catalysts (Ziegler-Natta and metallocene catalysts) can be used in the preparation of linear polyolefins, and it normally is difficult to incorporate functional group-containing monomers into the polyolefins by using the early transition metal catalysts due to catalyst poisoning (see J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, New York, 1979). The Lewis acid components (Ti, V, Zr and Al) of the catalyst will tend to complex with nonbonded electron pairs on N, O, and X (halides) of functional monomers in preference to complexation with the .pi.-electrons of the double bonds. The net result is the deactivation of the active sites by formation of stable complexes between catalysts and functional groups, thus inhibiting polymerization.
In several patents (see U. S. Pat. Nos. 4,734,472; 4,751,276; 4,812,529; 4,877,846), it has been taught to prepare borane-containing polyolefins. The chemistry disclosed in such patents involves a direct polymerization using organoborane-substituted monomers and alpha-olefins in Ziegler-Natta and metallocene processes. The homo- and copolymers containing borane groups are very useful intermediates for preparing a series of functionalized polyolefins. Many new functionized polyolefins with various molecular architectures have been obtained based on this chemistry. In addition, it has been demonstrated that polar groups can improve the adhesion of polyolefins to many substrates, such as metals and glass (see Chung et al, J. Thermoplastic Composite Materials, 6, 18, 1993 and Polymer, 35, 2980, 1994). The chemistry of borane-containing polymers also has been extended to the preparation of polyolefin graft copolymers, which involves free radical graft-from reaction (see U.S. Pat. Nos. 5,286,800 and 5,401,805). In polymer blends, compatibility of the polymers can be improved by adding a suitable polyolefin graft copolymer which reduces the domain sizes and increases the interfacial interaction between domains (see Chung et al, Macromolecules, 26, 3467, 1993; Macromolecules, 27, 1313, 1994).
Another approach toward preparing functionalized polyolefins is the preparation of unsaturated polyolefin copolymers containing pending unsaturated side chains which are reactive in subsequent chemical functionalization reactions. In general, the transition metal (Ziegler-Natta and metallocene catalysts) copolymerization of alpha-olefin and diene monomer is a great concern with many potential side reactions. The diene monomer, containing two reactive sites, potentially may engage in a double addition reaction to form copolymers having long branching side chains or even three dimensional network (crosslinked) structures. Most of linear diene-containing copolymers that have been reported involve the use of asymmetric dienes (see U.S. Pat. Nos. 3,658,770; 4,680,318; and 4,366,296) which contain only one polymerizable olefin unit, either an alpha-olefin or a constrained cyclolefin moiety, to prevent the formation of crosslinked (unprocessible) products. The asymetric dienes include those containing an alpha-olefin unit and an internal olefin unit, such as 1,4-hexadiene and methyl-1,4-hexadiene, and those containing a constrained cycloolefin unit and a linear olefin unit, such as 2-methylene-5-norborene, 5-vinyl-2-norborene and dicyclopentadiene. Several unsaturated polyolefins have been reported, including unsaturated polyethylene copolymers (Marathe et al. Macromolecules, 27, 1083, 1994), polypropylene copolymers (Kitagawa et al., Polymer Bulletin, 10, 109, 1983) and ethylene-propylene terpolymers (VerStrate et al, Encyclopedia of Polym. Sci. and Eng., 6, 522, 1986). Recently, Machida et al. (JP 05-194665 and JP 05-194666) also reported the copolymerization of alpha-olefins and asymetric styrenic diene comonomers, such as p-(3-butenyl)styrene, to produce linear copolymers using Ziegler-Natta heterogeneous catalysts.
Alpha-olefin polymerization involving symmetric alpha,omega-diene comonomers in which both double bonds are terminal alpha-olefins are very limited. One such polymerization, which involved the copolymerization of alpha-olefin and 1,3-butadiene (Bruzzone et al., Makromol. Chem., 179, 2173, 1978; Cucinella et al., European Polym. J., 12, 65, 1976), resulted in copolymers where the butadiene units in the copolymer were mostly in the trans-1,4-configuration. In other words, both alpha-olefins in the butadiene monomer were engaged in the polymerization reaction. Some diene comonomers having a long spacer between two terminal olefins, including C.sub.8 -C.sub.14 aliphatic alpha,omega-dienes, such as 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene and 1,13-tetradecadiene (see U.S. Pat. Nos. 4,551,503; 4,340,705; and 5,504,171), were found to be more selective so as to engage only one olefin group in the heterogeneous Ziegler-Natta copolymerization reaction. The resulting polyolefin copolymers have pending alpha-olefin groups located along the polymer chain.
Incorporating divinylbenzene (a symmetric alpha,omega-diene) comonomer into a linear polyolefin would result in polyolefin copolymers containing pending styrene groups, as illustrated below in Formula (I). Such copolymers could be used as versatile precursors for a broad range of polyolefin structures, including the polyolefin graft copolymers containing polyolefin backbone and other polymer side chains. However, it is very difficult to prepare linear polyolefin copolymers having a well-defined molecular structure, as illustrated in Formula (I), due to potential branching and crosslinking reactions, resulting from the difuntional nature of divinylbenzene.
The transition metal copolymerization of styrenic monomers and alpha-olefins usually is very difficult to accomplish (see Seppala et al., Macromolecules, 27, 3136, 1994 and Soga et al., Macromolecules, 22, 2875, 1989). This is especially true when using stereospecific heterogeneous Ziegler-Natta catalysts having multiple active sites, since the reactivity of monomer is sterically controlled, i.e., the larger the size of the monomer, the lower the reactivity; and those few styrenic copolymers that are known tend to be very inhomogeneous (Mijatake, et al., Makromol. Chem. Macromol. Synp., 66, 203, 1993; Aaltonen, et al., Macromolecules, 27, 3136, 1994; and U.S. Pat. No. 5,543,484) and to have broad molecular weight and composition distributions.
The copolymerization of alpha-olefin and divinylbenzene by Ziegler-Natta catalysts has been disclosed (Yokoyama, et al., Eur. Pat. Appl. 88310305.3 and Yoshitake, et al., JP 62-241907). It also has been disclosed that the resulting copolymers can be used in the preparation of polyolefin graft copolymers (Yokoyama, et al., JP 03-255114; Tomita, et al., JP 08-003231, JP 08-003232 and JP 05-017539). However, as expected, the known copolymers of divinylbenzene and alpha-olefins, especially ethylene and propylene, are very inhomogeneous, showing broad composition and molecular weight distribution (Mw/Mn&gt;6), due to multiple active sites and sterically-controlled monomer reactivity. Also, the extent of side reactions have not reported, possibly because it may be very difficult to determine the extent of side reactions due to the very low concentration of divinylbenezene in the copolymer products. The divinylbenzene content in the ethylene and propylene copolymers is below 0.3 mole % (1 wt %) and the overall divinylbenzene conversion is only few % in each case. In general, the catalyst activity is inversely proportional to the concentration of divinylbenzene in the monomer feed.
Machida, et al. (U.S. Pat. No. 5,608,009) also reported the copolymerization reaction of ethylene and diene comonomers (inlcuding divinylbenzene) by using metallocene catalysts. The diene-containing copolymers were used as intermediates in the preparation of graft copolymers, including long chain branching polyolefins. In general, the alpha-olefin/divinylbenzene copolymers reported by Machida, et al. were complex and had ill-defined molecular structures. Moreover, Machida, et al. failed to identify the reaction conditions that are necessary to prepare copolymers having a linear molecular structure and narrow composition and molecular weight distributions (as discussed in Column 16, lines 41-45, the olefin copolymers obtained by Machida, et al were long-branched copolymers). The disclosed examples of copolymerization reactions between ethylene and divinylbenzene involved using dicyclopentadienylzirconium dichloride (in Example I) and cyclopentadienylzirconium trimethoxide (in Example 3) as the catalyst system. The molecular structures of the resulting ethylene/divinylbenzene copolymers were complex and the copolymers were characterized by a low molecular weight (Mw=5,670 in Example 1 and Mw=14,500 in Example 3) and broad molecular weight distributions (Mw/Mn=6.6 in Example 1 and Mw/Mn=23 in Example 3). The inhomogeneous and non-linear copolymer structures were clearly revealed by the ratio of unsaturation/divinylbenzene (TUS/DOU) in the copolymers, the ratios being 0.71 (in Example 1) and 7.55 (in Example 3) using dicyclopentadienylzirconium dichloride and cyclopentadienyltitanium trimethoxide, respectively.
It is well known that metallocene polymerization results in polymers that are terminated mainly by beta-hydride elimination to form an unsaturated site at the chain end. Accordingly, it would be logical that the TUS/DOU ratio should be near unity for a linear copolymer of the type contemplated by the present invention, as illustrated in Formula (I). Thus, for a linear polymer, it would be expected that as the polymerization reaction continues and as the molecular weight increases (and as divinylbenzene units become incorporated into the copolymer), the contribution of chain end unsaturation to the TUS/DOU ratio would be very small. In other words, the TUS/DOU ratio should remain at or very close to unity. Similarly, it would be logical to assume that a copolymer that is characterized by a TUS/DOU ratio that deviates substantially from unity would be a non-linear, inhomogeneous copolymer containing many chain ends. For the known ethylene/divinylbenzene copolymers that were prepared using dicyclopentadienylzirconium dichloride (Example 1, above), the ratio of TUS/DOU=0.71 strongly suggests that a good portion of the divinylbenzene units that were incoporated into the copolymer had undergone double addition reactions at both vinyl groups to produce a polymer having a long-chain branching structure. Overall, the prior disclosures fail to identify the reaction conditions, especially the catalyst systems, that are necessary to prepare linear alpha-olefin/divinylbenzene copolymers having narrow composition and molecular weight distributions.
In general, the advances in metallocene catalysts (see U.S. Pat. Nos. 4,542,199; 4530,914; 4,665,047; 4,752,597; 5,026,798 and 5,272,236) provide an excellent opportunity for chemists to prepare new polyolefin polymers. With well-defined (single-site) catalysts and a designed active site geometry, monomer insertion can be controlled effectively, both kinetically and sterically, during a polymerization process. This is especially important for copolymerization reactions for producing copolymers having a relatively well-defined molecular structure. Several prior publications have disclosed the use of metallocene catalysts having a constrained ligand geometry for producing narrow composition distribution and narrow molecular weight distribution linear low density polyethylene (LLDPE).
For copolymerization reactions, use of a relatively opened active site metallocene catalyst provides essentially equal access for both comonomers, and the incorporation of higher molecular weight olefin comonomer is significantly higher than for those copolymers obtained from tranditional Ziegler-Natta catalysts. In fact, some metallocene catalysts with constrained ligand geomerty and opened active site have shown to be effective for incorporation of styrenic monomers in polyolefin copolymers, including poly(ethylene-co-styrene) (U.S. Pat. No. 5,703,187) and poly(ethylene-co-p-methylstyrene), poly(ethylene-ter-propylene-ter-p-methylstyrene) and poly(ethylene-ter-1-octene-ter-p-methylstyrene) (U.S. Pat. No. 5,543,484, and J. Polym. Sci. Polym Chem., 36, 1017, 1998, Macromolecules, 31, 2028, 1998).